Currently, the directory URLs are like that: Option A -> URL/country/district/sub-district/town

It is nice and easy to remember but it may too deep for SEO.

Another option could be: Option B -> URL/country_district_sub-district_town/ Less nice looking, but stay close to root directory

Lastly, I was also thinking to have this option Option C -> URL/town/ OR Option C -> URL/country/ Very easy to remember, but a town can have many matches (like same town but in two different sub-districts)

What would be the best option between A, B and C for Google optimisation with Adsense use?

Option A sounds like the best to me, purely because it seems to reflect the information hierarchy better. Of course, you would need meaningful content at each 'subdirectory' index page for everything to work.

too deep for SEO

Too deep really means 'not linked to often enough' rather than how many slashes are in the URL.